1. Field of the Invention
The invention relates to an ink jet recording instrument in accordance with the preamble of claim 1.
As compared to recording instruments using lever arms, ink jet recording instruments for the analog recording of measuring magnitudes in the form of amplitude curves, such as in medical technology in connection with EEG and EKG devices, exhibit the advantage of a relative absence of mass inertia, so that frequencies up to 1200 Hertz can be recorded. Moreover, such ink jet recording instruments operate noiselessly and their range of amplitudes is not limited to a predetermined writing width, inasmuch as the recorded amplitude curves may overlap one another.
2. Description of the Prior Art
In German Pat. No. 821,065 there is disclosed an ink jet recording instrument in which the capillary tube serving for supplying and conveying the writing liquid, i.e. the ink, from a supply tank via a pump to the writing nozzle, is connected to a loop of silver wire by means of two parallel lengths of wire, whereby the silver wire loop is disposed within the magnetic field of a permanent magnet. The bent writing jet exit nozzle of the capillary tube is angularly displaced as a function of an electric current through the wire loop which current is controlled by the measuring magnitude to be recorded. This jet recording instrument, which has been known for a relatively long time, takes up too much space for most of the contemporary requirements because of the relatively large permanent magnet, as it is frequently desirable to generate a plurality of families of curves in a side-by-side arrangement along the same time axis, which requirement makes it necessary to mount several of such writing mechanisms one closely adjacent to another. This known liquid jet recording instrument is, furthermore, also quite complicated as far as its construction is concerned.
The published German Application No. 26 19 369 describes a liquid jet recording instrument of more contemporary construction, wherein a small, diametrically magnetized permanent magnet is comented to the capillary tube at a location ahead of its bent end which forms the exit nozzle for the writing liquid, with the permanent magnet being disposed between two pole shoes made of soft iron. The electromagnet with which the two pole shoes are connected has the shape of two tubelets which coaxially encompass the capillary tube, the energizing coil being disposed between the tubulets in order to achieve a space-saving arrangement. In this known arrangement, the permanent magnet, which is connected to the end of the capillary tube and disposed between the poles of the electromagnet, is mounted within a tube which is filled with a damping liquid, the tube being closed at either one of its ends by bearings for the capillary tube.
As compared to the first-mentioned type of liquid jet recording instruments, the second-mentioned recording instrument has the advantage that it can be produced as a compact structure, thus permitting the arrangement of a plurality of writing mechanisms mounted closely one to the other. There are, however, extremely strict requirements with respect to manufacturing tolerances and this is applicable not only to the step of mounting the capillary tube into the external tube which is provided with bearings, but this is also applicable to the step of cementing the permanent magnet to the capillary tube. The capillary tubes used in such jet recording instruments are extremely thin glass tubes whose external diameter falls within the order of magnitude of the diameter of a human hair. In order to guarantee the optimum of identical tolerances in connection with all recording mechanisms of a multiple recording instrument, very strict manufacturing tolerances not only apply to the step of pulling the capillary tubes, but also particularly to the step of cementing the permanent magnet to the bent end of the capillary tubes. A further disadvantage of basic importance, which prevails also in connection with liquid jet recording instruments in accordance with the first-mentioned German patent, resides in the fact that the capillary tube is not only used for conveying the recording liquid from a pump for the liquid, but also that the capillary tube, upon torsion, i.e. a twisting effect, applied to it, supplies the mechanical restoring force of the writing mechanism. As the capillary tube thus operates not only as the ink supply channel but also as the return spring, it determines, in association with a permanent magnet, in essence, the frequency limit, so that it is necessary that certain properties of the glass of the capillary tube are maintained constant along the entire length of the capillary tube. As a result, it becomes extremely difficult to adjust the driving system of the recording instrument.
In spite of these narrow manufacturing tolerances, only the measuring quality of class 2.5 can be achieved with such ink jet recording instruments, i.e. the recorded measuring values include errors, i.e. deviations, of 2.5% with respect to the final amplitude, i.e. the maximum amplitude. In contrast thereto, lever arm recording instruments permit achieving quality classes of 0.5 for example.
Disregarding the difficulties which result from the double function of the glass capillary tube, namely its function as an ink supply channel and as a restoring spring, an essential cause of the relatively unsatisfactory quality class resides in the fact that there is a substantial length of cantilevered writing nozzle which extends beyond the driving magnet, and therefore, the nozzle can not be well conducted or supported in the direction of the jet downstream from the permanent magnet, so that the bearing at the writing end of the capillary tube actually serves merely as a stop element for preventing wobbling motion of the writing nozzle. The thus resulting relatively great distance between the location of drive or rotation, which is determined by the position of the permanent magnet, and the location where the capillary tube is bent at right angle to form the exit nozzle is, therefore, an essential cause for the unsatisfactory lack of accuracy in connection with such known ink jet recording instruments.